The present application relates to signal processing apparatus.
In particular, this application relates to signal processing apparatus in which an input signal is subject to intermodulation distortion (IMD) between its input and output. Amplifying circuits, such as those used in transmitters and receivers, can constitute signal processing apparatus of this type.
For example, an RF power amplifier (PA) can, under certain operating conditions, provide an output signal which is a distorted version of its input signal. The distorted output signal can contain IMD products 110, 112 (FIG. 1) appearing around the output components 114, 116 corresponding to the input spectrum 100.
It is known to compensate for IMD distortion by pre-distorting the input signal spectrum to signal processing apparatus such as a non-linear PA. One form of predistortion generator produces a predistortion comprising controlled quantities of higher order versions of the input signal and introduces it into the input signal prior to the latter""s supply to the signal processing means suffering from distortion. In this type of predistorter, the predistortion can contain, for example, third order components created by cubing the input signal and fifth order components created by forming the product of the cube and the square of the input signal.
However, the type of predistorter just discussed does not satisfactorily linearise non-linear signal processing apparatus which inflicts unequal IMD on the input signal. FIG. 1 shows an example of an output spectrum of a PA exhibiting unequal IMD of an input spectrum comprising a pair of spaced tones of equal level (represented in the output spectrum as 114 and 116). The IMD components 110 and 112 have different levels and therefore their cancellation using the predistortion mechanism described above is less effective.
Most (but not all) quasi-linear (class A or AB) RF power amplifiers will exhibit a well-behaved IMD characteristic at narrow tone spacing values (e.g. a few kHz) and this may continue up to a few megahertz or even tens of megahertz in some carefully-designed amplifiers. However, if one considers the previous example of two input tones having the same level, the majority of amplifiers will begin to exhibit a varying degree of inequality between IMD products of the same order as the spacing of the input tones is increased.
There is a wide range of reasons why the IMD products are unequal, and these include AMxe2x80x94AM and AM-PM distortions within the amplifying device being applied at different phases and hence resulting in a degree of subtraction of one or other product (usually accompanied by a degree of addition to other product); imperfect power supply or bias decoupling resulting in baseband (tone-difference) modulations being applied to the signalsxe2x80x94this modulation will almost always be partially out-of phase with the IMD products, hence causing the partial cancellation/addition outlined above; and reflection of unterminated harmonic distortion components from e.g. output combining networks causing additional (and again out-of-phase) IMD products to be generated in the amplifier output.
According to one aspect, the invention consists in distorting apparatus for altering an input signal to signal processing means so as to counter unequal intermodulation distortion (IMD) in the output signal of the signal processing means to make it more linear.
Advantageously, the distorting apparatus comprises generating means for generating a distortion signal from the input signal and comprises two parallel signal paths, which each receive the distortion signal, and compensating means located in at least one of the paths, the compensating means introducing a phase and/or amplitude variation with frequency into the distortion signal, and combining means for recombining the signals issuing from the two signal paths.
Preferably the compensating means is filter means or delay means.
In a preferred embodiment, the filter means is a digital filter implemented by a digital signal processor (DSP), and the filter""s characteristics, are adapted in response to a feedback signal derived from the output signal of the signal processing means which is indicative of IMD components present therein.